fm_transmitter

FM Transmitter

Use the Raspberry Pi as an FM transmitter. Works on Raspberry Pi boards up to 4 (RPi 5 uses different peripherial chip).

Just get an FM receiver, connect a 20 - 40 cm plain wire to the Raspberry Pi's GPIO4 (PIN 7 on GPIO header) to act as an antenna, and you are ready for broadcasting.

This project uses the general clock output to produce frequency modulated radio communication. It is based on an idea originally presented by Oliver Mattos and Oskar Weigl at PiFM project.

Installation and usage

To use this software you will have to build the executable. First, install required dependencies:

sudo apt-get update
sudo apt-get install make build-essential

After installing dependencies clone this repository and use make command in order to build executable:

git clone https://github.com/markondej/fm_transmitter
cd fm_transmitter
make

After a successful build you can start transmitting by executing the "fm_transmitter" program:

sudo ./fm_transmitter -f 100.6 acoustic_guitar_duet.wav

Notice:

• -f frequency - Specifies the frequency in MHz, 100.0 by default if not passed
• acoustic_guitar_duet.wav - Sample WAV file, you can use your own

Other options:

• -d dma_channel - Specifies the DMA channel to be used (0 by default), type 255 to disable DMA transfer, CPU will be used instead
• -b bandwidth - Specifies the bandwidth in kHz, 100 by default
• -r - Loops the playback

After transmission has begun, simply tune an FM receiver to the chosen frequency, and you should hear the playback.

Raspberry Pi 4

On Raspberry Pi 4 other built-in hardware probably interfers somehow with this software making transmitting not possible on all standard FM broadcasting frequencies. In this case it is recommended to:

1. Compile executable with option to use GPIO21 instead of GPIO4 (PIN 40 on GPIO header):

make GPIO21=1

1. Changing either ARM core frequency scaling governor settings to "powersave" or changing ARM minimum and maximum core frequencies to one constant value (see: https://www.raspberrypi.org/forums/viewtopic.php?t=152692 ).

echo "powersave"| sudo tee /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

1. Using lower FM broadcasting frequencies (below 93 MHz) when transmitting.

Use as general audio output device

hydranix has came up with simple method of using transmitter as an general audio output device. In order to achieve this you should load "snd-aloop" module and stream output from loopback device to transmitter application:

sudo modprobe snd-aloop
arecord -D hw:1,1,0 -c 1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 - &

Please keep in mind loopback device should be set default ALSA device (see this article). Also parameter "-D hw:X,1,0" should be pointing this device (use card number instead of "X").

Microphone support

In order to use a microphone live input use the arecord command, eg.:

arecord -D hw:1,0 -c 1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 -

In cases of a performance drop down use plughw:1,0 instead of hw:1,0 like this:

arecord -D plughw:1,0 -c 1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 -

Supported audio formats

You can transmitt uncompressed WAV (.wav) files directly or read audio data from stdin, eg. using MP3 file:

sudo apt-get install sox libsox-fmt-mp3
sox example.mp3 -r 22050 -c 1 -b 16 -t wav - | sudo ./fm_transmitter -f 100.6 -

Please note only uncompressed WAV files are supported. If you receive the "corrupted data" error try converting the file, eg. by using SoX:

sudo apt-get install sox libsox-fmt-mp3
sox example.mp3 -r 22050 -c 1 -b 16 -t wav converted-example.wav
sudo ./fm_transmitter -f 100.6 converted-example.wav

Or you could also use FFMPEG:

ffmpeg -i example.webm -f wav -bitexact -acodec pcm_s16le -ar 22050 -ac 1 converted-example.wav
sudo ./fm_transmitter -f 100.6 converted-example.wav

Legal note

Please keep in mind that transmitting on certain frequencies without special permissions may be illegal in your country.

New features

• DMA peripheral support
• Allows custom frequency and bandwidth settings
• Works on every Raspberry Pi model
• Reads mono and stereo files
• Reads data from stdin

Included sample audio was created by graham_makes and published on freesound.org